The present invention relates to an organic photoreceptor for use in an image formation of an electro-photographic method, an image forming apparatus employing the organic photoreceptor, an image forming method and a process cartridge and in more detail, to an organic photoreceptor for use in an image formation of an electro-photographic method usable in the field of a copying machine and a printer, an image forming apparatus employing the organic photoreceptor, an image forming method and a process cartridge.
The main subject of a photoreceptor is transferred from an inorganic photoreceptor such as Se, arsenic, arsenic/Se alloy, CdS, ZnO, to an organic photoreceptor which has advantages in the environmental pollution, or easiness of manufacturing, and the organic photoreceptors using various materials are developed.
Recently, the function separation type photoreceptor in which functions for generating the electronic charge and for charge transportation are made in charge to different materials, becomes the main stream, for example, a laminated type photoreceptor in which the charge generation layer, charge transporting layer are laminated through the intermediate layer on the conductive supporting body, is widely used (Patent Document 1).
On the other hand, even for a copying machine and a printer, there is a strong demand for digital color images having high image quality. For the demand of this kind, it has been proposed that a laser beam having a short wavelength is used as a light source for exposure, and thereby, a digital image of high density is formed (Patent Document 2). However, even when an electrostatic latent image is formed on an organic photoconductor by using a laser beam having a short wavelength, it is not always possible to form a microscopic dot latent image, and that dot latent image cannot be reproduced separately and at high density, and an individual dot image cannot be reproduced independently, or it disappears, thus, a minute electrophotographic image has not been attained sufficiently.
For reproducing a highly fine electrostatic latent image faithfully, potential contrast between an exposed portion and an unexposed portion needs to be secured sufficiently, and for that purpose, it is important to restrain diffusion of carrier during a period for carrier generated by image-wise exposure to arrive at charges on the surface of a photoconductor. With respect to deterioration of a latent image for a high density image, an effect of diffusion to an electrostatic latent image cannot be ignored when a ratio D/μ of a diffusion constant of a charge transport layer (D) to drift mobility (μ) grows greater, and it is reported that deterioration of a latent image grows greater when a film thickness of a charge transport layer is great. (Non-patent Document 1).
An electrophotographic photoconductor wherein diffusion of an electrostatic latent image is prevented by making a film of a charge transport layer to be thin, has already been proposed in the field of a patent. However, when an image is formed on the proposed organic photoconductor by the use of an electrophotographic image forming apparatus actually, a blurred image with lowered image density tends to appear. The reason for this is as follows; if a film of an electrophotographic photoconductor is made to be thin, capacitance of a photosensitive layer becomes smaller, charging potential tends to be lowered and sufficient image potential (a difference between the charging potential and residual potential) is hardly obtained, resulting in a decline of developability and a decline of image density. In particular, when using a developer employing small-sized toner particles for obtaining high image quality, developability is lowered, image density is not sufficient in reversal development, and it is difficult to obtain character images having excellent sharpness and photographic images, which is a problem.
For restoring the decline of developability, it is effective to give charges wherein an amount of charges per unit area is increased to the electrophotographic photoconductor with a thin film, and thereby, to increase electric field strength per unit film thickness of the photoconductor. However, when the electric field strength per unit film thickness of the photoconductor is increased, sensitivity shortage of the photoconductor is caused, residual potential tends to be increased in the course of repeated image forming operations, and a decline of image density and periodic image defects such as a black spot and white clearness tend to be caused.
(Patent Document 1) TOKUKAI No. 2004-133018
(Patent Document 2) TOKUKAIHEI No. 5-119503
(Non-patent Document 1) Page 296, No. 4 of Volume 38 in Journal of Imaging Society of Japan